1. INTRODUCTION TO COOLINGIN OUR PREVIOUS CLASSES WE HAVE LEARNT ABOUT KINETICTHEORY OF GAS MOLECULES ACCORDING TO WHICH T = 2 __K___ 3 N.KbWHERE T IS TEMPERATURE OF THE MOLECULE K IS KINETIC ENERGY OF GAS MOLECULES N IS NUMBER OF MOLECULES Kb IS BOLTZMANN CONSTANT HERE WE CAN CLEARLY SAY THAT THETEMPERATURE IS DIRECTLY PROPORTIONAL TO KINETIC ENERGY OFTHE GAS MOLECULE SO BASICALLY THE COOLING IS SOMETHINGWHICH HAPPENS DUE TO DECREASE IN THE KINETIC ENERGY OF THEMOLECULES IN THE PRESENT SAMPLE OF GASES . IF WE TAKE EXAMPLE IN OUR EVERY DAY LIFE WHEN WE BOILWATER THE KINETIC ENERGY OF THE WATER MOLECULESINCREASES AS A RESULT THE TEMPERATURE OF THE WATER SAMPLEINCRESES. THIS IS THE BASIC PRINCIPLE WHICH WE ARE GOING TO APPLYINTO OUR PROJECT.

2. COOLING USING LASERSLASER COOLING REFERS TO THE NUMBER OF TECHNIQUES IN WHICHATOMIC AND MOLECULAR SAMPLES ARE COOLED THROUGH THEINTERACTION WITH ONE OR MORE LASER LIGHT FIELDS. THERE ARE MANY TECHNIQUES WHICH ARE USING THIS PRINCIPLE FOR COOLING THE GAS BY APPLYING LASERS FROM DIFFERENT DIRECTIONS. SOME EXAMPLES ARE Doppler cooling Sisyphus cooling Resolved sideband cooling Velocity selective coherent population trapping (VSCPT) Anti-Stokes inelastic light scattering (typically in the form of fluorescence or Raman scattering) Cavity mediated cooling Sympathetic cooling Laser cooling is primarily used for experiments in Quantum Physics to achieve temperatures of near absolute zero (273.15C, 459.67F). This is done to observe the unique quantum effects that can only occur at this heat level. Generally, laser cooling has been only used on the atomic level to cool down

3. DOPPLER COOLINGBrief ExplanationDoppler cooling involves light whose frequency is tuned slightly belowan electronic transition in an atom. Because the light is detuned to the"red" (i.e. at lower frequency) of the transition, the atoms will absorbmore photons if they move towards the light source, due to theDoppler effect. Thus if one applies light from two opposite directions,the atoms will always absorb more photons from the laser beampointing opposite to their direction of motion. In each absorptionevent, the atom loses a momentum equal to the momentum of thephoton. If the atom, which is now in the excited state, emits a photonspontaneously, it will be kicked by the same amount of momentum butin a random direction. The result of the absorption and emissionprocess is a reduced speed of the atom, provided its initial speed islarger than the recoil velocity from scattering a single photon. If theabsorption and emission are repeated many times, the mean velocity,and therefore the kinetic energy of the atom will be reduced. Sincethe temperature of an ensemble of atoms is a measure of the random

4. Some Questions which Need to be Answered 1. Usually light incident on any matter justincreases its temperature but in this case lasers arecooling it down how?Ans. The basic theory used behind this is let thephoton bounce back from the atom for ex. If wehave a big rubber ball rolling we can hit it with smallrubber balls and can slow down the speed of thebig ball by transferring the momentum. In this caselaser lights photon is the smaller rubber ball and theatom is the big rubber ball which is bouncing thephotons from the surface.Following scientists got Nobel Price in field ofphysics for discovering the cooling principle.

5. Born: 28 February 1948, St. Louis, MO,USAAffiliation at the time of the award:Stanford University, Stanford, CA, USAPrize motivation: "for development ofmethods to cool and trap atoms with laserlight"Field: Atomic physicsBorn: 1 April 1933, Constantine, AlgeriaAffiliation at the time of the award:Collge de France, Paris, France, coleNormale Suprieure, Paris, FrancePrize motivation: "for development ofmethods to cool and trap atoms with laserlight"Field: Atomic physics

6. Born: 5 November 1948, Wilkes-Barre, PA, USA Affiliation at the time of the award: National Institute of Standards and Technology, Gaithersburg, MD, USA Prize motivation: "for development of methods to cool and trap atoms with laser light" Field: Atomic physicsfigure to describe the change in the momentum of the atom Source-http://www.nobelprize.org

7. 2.But in case of atom it is not a true replica of theex. Given in the above question then how we can use it?Ans. Actually the atom is absorbing the photon and thenin fraction of seconds it is releasing it back with moremomentum as it is also adding some of its momentum asthey are in continuous vibration. The time betweenabsorbing and releasing is such low that it is negligibleand we can consider it as bouncing of photon form theatom. Photon absorption and emission

8. 3. The next thing is the atom require a photon of aparticular wavelength or better say of particular colour toexcite the atom so how it is done ?Ans. This thing is done By making a light source ofvarying frequency such that we can adjust it to aparticular frequency as per the requirement. But even avibration in the ground can effect the frequency of theemitted light, we have to consider this also. HERE THE RUBIDIUM ATOM IS EXCITED BY USING LASER LIGHT THE THING TO BE NOTED IS THAT TRANSITON IS STRAIGHT LIKE IF A-B THEN B-A NOT B-C WHERE C IS INTERMEDIATE BETWEEN A AND B.

9. 4. The laser slow down the fast moving atoms but itshould also increase the speed of the slower moving atomwhy it is not doing so?Ans. This question was one of the most difficult questionwhich almost made this theory a waste but some cleverscientist solved this using a simple theory which isDoppler effect and as the frequency changes with thechange in the position of the light source and atom theatom neglects the most of the frequency and thats howthey are able to affect only the atoms with higher vibratingspeed. This was the basis of the Doppler Shift called THE FOLLOWING FIGURE SHOWS THE DOPPLER SHIFT HAPPENING INhence the cooling is also known as DopplerDUE TO THE RESONANCE cooling. MOVEMENT OF THE ATOM

10. 5. The previous answer was applicable for only onedirection what about the atoms which are moving inrandom direction?Ans. Answer to this question was also very easy as to uselaser from different directions so a machine was builtwhich was able to throw the laser to the sample from360*360 deg. Angle. So it was like a sphere of small-smalllaser sources bounded together. Cooling the atoms andgetting a bunch of cooled atoms is called as "optical HERE THE ATOM IS BEEN STRIKED BY LASEmolasses SOURCES FROM ALL ROUND THE DIRECTIO AS PER THE ABOVE QUESTION IS ANSWERE

11. 6.Now its ok with cooling but what about the atomsmoving in different directions they are not controlled asthey can hit the wall of the container and can gain theheat.Ans. This problem was also a tricky one but the answerthe scientists got was very easy as they use strongmagnets to control them as the atom are also tinnymagnets they respond to the magnetic field and getattracted toward them they applied it from all thedirections to get the atoms in the centre of the container HERE THE ATOMS ARE KEPT IN Aand not allowing them to hit the FIELD SO THAT THEY ARE KEPT MAGNETIC walls. IN THE CENTRE OF THE CHEMBER.

12. 7. Now what if the atoms in between themselves onlyhit each other and get heated up.Ans. For this the machine is having vacuum pumpsso there are no any excess air atoms also theamount of atom taken in the container is very low asit would not allow the atoms to hit each other.

13. LASER COOLINGARRANGEMENT

14. WORK OF DIFFERENT PARTS 1- Frequency stabilised laser diode. 3-Two lens telescope arrangement which expands the laser beam. 4,6,7,9,10,11- divide the incident beam into three beams of nearly equal width and equal intensity travelling in the three mutually orthogonal directions in the trapping cell. 13,14,15 and 16 -The beam along the axis of the magnetic coils should be circularly polarized in the opposite direction which is achieved by setting the axes of the four quarter wave plates 13, 14, 15 and 16.

15. 5,8 and 12- Mirrors which retro-reflect the circularly polarized light beams along the three orthogonal directions after they pass through the quarterwave plates(14,16 and 18). 20 Laser diode which produces frequency stabilised hyperfine laser beam. 23a and 23b Pair of mirors. 24 and 25 Circular coils which produce a quadrupolar magnetic field